1. Field of the Invention
The present invention relates to a device and method for identifying a security or bank note (hereinafter bank note) which can easily separate a genuine bank note from a false one.
2. Description of the Related Art
Recently, use of various types of vending machines has rapidly proliferated handling all sorts of merchandise. Certain types of vending machines have been introduced which can handle not only coins but also bank notes. Since vending machines are set up in various locations and undergo diversified operational conditions, they are required to be able to perform satisfactorily under all sorts of operational environments. This also applies to mechanisms for distinguishing genuine coins and/or bank notes from false ones.
A mechanism for identifying genuine coins can be constructed by a mechanical system, as false coins can be separated from genuine ones by checking their weights and/or shapes.
On the other hand, it has been almost impossible to recognize a bank note as genuine or false by a mechanical system. Rather, identification has been conducted by optically checking the extent of light transmission through a bank note. In a conventional way of bank note identification a light source and a light receiving element are positioned with specified distance between them to transport a bank note therebetween for detecting a bright-dark pattern peculiar with regard to the light source to compare with a reference bright-dark pattern stored in advance for determination of genuine or false.
However, in such a case, there existed a likelihood of easily mistaking a copy of a bank note as a genuine one, since identification of genuine or false has been done only by means of a bright-dark pattern.
Use of a color sensor may be proposed to improve this point but it is impossible to use such an expensive device as a color sensor, which requires complicated signal processing for a vending machine for which low production cost is an essential factor.
Further, it is required to use an incandescent light source (incandescent bulb) but an incandescent light source tends to be short lived and especially when a vending machine is set up on a roadside under hot summer sun, it is exposed to strong light and heat which causes a breakdown of a bulb in a short while. In such a case even a genuine bank note is mistaken as false when it is inserted into such a vending machine making it useless.
To solve those disadvantages, it may be proposed to determine color tint of a bank note by employing, for example, 2 light emitting diodes (LED), which emit lights in different wavelengths as light sources and a light receiving element which receives light emitted from each of the LEDs. However, as LEDs have diversified light emission characteristics, it was necessary to adjust set-up positions and drive currents of LEDs to obtain a uniform performance ratio between 2 LEDs.
Further, in case of a vending machine which is supposed to be set up in the outdoors, it is difficult to maintain a constant ratio of intensity of light emitted by LEDs. This totally applies to a light receiving element. This resulted in unreliable performance of a bank note identification device and it was not suitable for actual usage. To solve such disadvantages, disclosed in Japanese Examined Patent Publication No. 58-9478 is a device which is provided with 2 LEDs, one of them being green, while the other is red, lights emitted from the 2 LEDs being received by a light receiving element and the quantity of light emitted by the 2 LEDs being controlled so as to be equal, when there is no bank note between the LEDs and a light receiving element. When a bank note comes between the LEDs and the light receiving element, an error signal is to be generated, either if an output of the light receiving element derived from the light of green LED takes a higher value than a reference level indicating the color of the bank note is inclined toward green in certain degrees or if the output of the light receiving element derived from the light of the red LED takes a higher value than the reference level, indicating the color of the bank note is inclined toward red in certain degrees.
However, the device in Japanese Examined Patent Publication No. 58-9478 is based on a premise that when the color of a bank note is not inclined (not shifted) toward red or green, the quantity of light received by the light receiving element put its output at zero level, which is totally equal to the output level found when there is no bank note. It can only generate an error signal in identifying such a bank note, when the color of a bank note is shifted toward green or red in certain degrees and it can be expected to have some effect only for a bank note printed in certain colors but it was almost impossible to identify bank notes printed in many colors such as Japanese paper money.
For example, it can not generate an error signal when a bank note is printed or copied in monochrome, making it useless under actual circumstances.